Potassium titanyl phosphate and certain of its analogs are non-linear optical materials, and are being investigated for a number of non-linear optical applications (see e.g., U.S. Pat. No. 3,949,323). Applications range from frequency doubling the 1 .mu.m radiation of Nd lasers to various electro-optic applications, such as modulators and Q switches. The use of these materials for forming waveguides is illustrated in U.S. Pat. No. 4,740,265 and U.S. Pat. No. 4,766,954. Recently, low loss optical waveguides of KTP have been developed for integrated-optic applications. Most of the above applications are based on the use of single crystal KTP.
Processes for producing optical quality crystals of KTiOP.sub.4 (KTP) and its analogs using aqueous systems are known generally in the art as hydrothermal processes (see e.g., U.S. Pat. No. 4,305,778) and processes for producing such crystals using nonaqueous molten salt systems are known generally in the art as flux processes (see e.g., U.S. Pat. No., 4,231,838). Typically, hydrothermal processes are run at elevated temperature and pressure and involve growing the crystal in a vessel having a growth region where the crystal grows and a nutrient region containing nutrient for growing the crystal, and employ an aqueous mineralization solution. U.S. Pat. No. 5,066,356 describes a hydrothermal process for growing optical-quality single crystals at reduced temperature and pressure conditions.
U S. Pat. No. 5,039,187 discloses a process for the liquid phase epitaxial growth of a thin film of certain KTP analogs on a substrate.
Sol-gel technology is well known in the art for fabricating a wide variety of glasses, fibers and films. The technique generally involves forming a solution of predetermined hydrolyzable components, at least partially hydrolyzing the components by the addition of water, then heat treating the solution to drive off the resulting alcohols, residual solvent and water to form a shaped, substantially homogeneous solid composition. Successful sol-gel processes typically offer the advantage of allowing use of relatively pure precursor materials, and selectively low temperatures, while providing a relatively homogeneous mix of components (see e.g., D. C. Bradley, Chem. Rev., 89, pp. 1317-1322 (1989)). The most common material to which this technology has been successfully applied is silica. Porous silica glass made by sol-gel methods has been further treated with various dopants, prior to hydrolysis and densification to form optical waveguides (Kondo et. al., Japanese Journal of Applied Physics, Part 1, Vol. 29, No. 12, pp. 2868-2874 (Dec. 1990)).
The majority of known sol-gel systems involve utilization of only one hydrolyzable components in the starting solution (see, e.g., L. L. Hench et. al., Chem, Rev 90(1), pp. 33-72 (1990)). Such systems allow for relatively predictable chemistry and manipulation of the reaction conditions. Multicomponent sol-gel systems (i.e., those processes using more than one hydrolyzable component in the starting solution) are less common. (See, e.g., U.S. Pat. No. 4,946,710.) Generally, the difficulty in achieving homogeneous products increases with the number of components.
Sol-gel processes have been developed for producing thin films of certain compositions. (See, e.g., U.S. Pat. No. 4,957,725.) The preparation of LiNbP.sub.3 films by a sol-gel process is described in D. P. Partlow et. al., J. Mater. Res., 2(5), pp. 595-605 (1987).